Currently, bipolar transistors are utilized in a myriad of high frequency communications applications. Many bipolar transistors feature a buried subcollector to assist in the collection of carriers while reducing series resistance. See e.g. U.S. Pat. No. 5,286,997, “METHOD FOR FORMING AN ISOLATED, LOW RESISTANCE EPITAXIAL SUBCOLLECTOR FOR BIPOLAR TRANSISTORS,” issued Feb. 15, 1994 to Hill and assigned to Texas Instruments.
More recently, higher switching speeds have been achieved by utilizing heterojunction bipolar transistors (HBT's), in which bipolar transistor elements are formed in graded SiGe or SiGeC layers that enhance carrier mobility. See e.g. U.S. Pat. No. 5,633,179, “METHOD OF FORMING SILICON/SILICON-GERMANIUM HETEROJUNCTION BIPOLAR TRANSISTOR,” issued May 27, 1997 to Kamins et al. See e.g. U.S. Pat. No. 5,506,427, “HETEROJUNCTION BIPOLAR TRANSISTOR WITH SILICON-GERMANIUM BASE,” issued Apr. 9, 1996 to Imai and assigned to NEC Corporation.
Another alternative that has developed is BiCMOS, which attempts to combine the high frequency operations of bipolars with the low power attributes of CMOS. See e.g. U.S. Pat. No. 5,856,695, “BICMOS DEVICES,” issued Jan. 5, 1999 to Ito et al., and assigned to Harris Corporation.